Problem with each Navon Classic T20

A lot of Classic T20 works well with the "0" boot key,but there are many T20 classic that are not bootable and this is the result:
Searching phone;
For an successful connection do the following:
1. Remove battery and usb cable
2. Insert the usb cable in the phone
3. Keep pressed the boot key and insert the battery
Phone detected (SCI USB2Serial)
Preparing to sync;
Port opened [COM6];
Mobile type: SPRD3
Connected;
Starting;
FDL sent;
Syncing;
ERROR: Sync failed;

Can someone solve it?